(a) Schematic circuit of the RCSJ model describing the dynamics of a Josephson junction. The bias current (I) is distributed among the three branches. (b) Tilted washboard potential. When the “particle” is in a potential minimum, it corresponds to a zero-voltage state. Increasing the bias current (I) increases the tilt, allowing the particle to escape and generates a finite voltage across the junction. (c) Quantization of the junction’s energy levels is analogous to the quantization of a particle in a quantum well. The system’s state can be changed by microwave irradiation. According to quantum mechanics, there is also a finite probability of escape via macroscopic quantum tunneling (MQT). Both properties were demonstrated in the experiments by Clarke, Devoret, and Martinis in the mid-1980s.